1. Field of the Invention
This invention relates generally to improved thermal spray application devices, and particularly to a feedstock injector for injecting feedstock material axially into a downstream flow of heated gas.
2. Description of Related Art
Thermal spraying may generally be described as a coating method in which powder or other feedstock material is fed into a stream of energized gas that is heated, accelerated, or both. The feedstock material is entrapped by the stream of energized gas from which it receives thermal and/or kinetic energy. The energized feedstock is then impacted onto a surface where it adheres and solidifies, forming a relatively thick thermally sprayed coating by the repeated cladding of subsequent thin layers.
It has been previously recognized that, in the case of some thermal spray applications, injecting feedstock axially into an energized gas stream presents certain advantages over other feedstock injection methods. Typically, feedstock is fed into a stream in a direction generally described as radial injection, in other words in a direction more or less perpendicular to the direction of travel of the stream. Radial injection is commonly used as it provides an effective means of mixing particles into an effluent stream and thus transferring the energy to the particles in a short span. Such is the case with plasma where short spray distances and high thermal loading require rapid mixing and energy transfer for the process to apply coatings properly. Axial injection can provide advantages over radial injection due to the potential to better control the linearity and the direction of feedstock particle trajectory when axially injected. Other advantages include having the particulate in the central region of the effluent stream, where the energy density is likely to be the highest, thus affording the maximum potential for energy gain into the particulate. Lastly axial injection tends to disrupt the effluent stream less than radial injection techniques currently practiced.
Thus, in many thermal spray process guns, axial injection of feedstock particles is preferred to inject the particles, using a carrier gas, into the heated and/or accelerated gas simply referred to in this disclosure as effluent. The effluent can be plasma, electrically heated gas, combustion heated gas, cold spray gas, or combinations thereof. Energy is transferred from the effluent to the particles in the carrier gas stream. Due to the nature of stream flow and two phase flow, this mixing and subsequent transfer of energy is limited in axial flows and requires that the two streams, effluent and particulate bearing carrier, be given sufficient time and travel distance to allow the boundary layer between the two flows to break down and thus permit mixing to occur. During this travel distance, energy is lost to the surroundings through heat transfer and friction resulting in lost efficiency. Many thermal spray process guns that do utilize axial injection are then designed longer than would normally be required to allow for this mixing and subsequent energy transfer to occur.
These limitations to mix the particulate bearing carrier and effluent streams becomes even more pronounced when the particulate-bearing carrier fluid is a liquid, and, in many cases, they have prevented the use of liquid feeding into axial injection thermal spray process guns. For liquid injection techniques the use of gas atomization to produce fine droplet streams aids in getting the liquid to mix with the effluent stream more readily to enable liquid injection to work at all but this method still requires some considerable distance to allow the gas and fine droplet stream and effluent stream to mix and transfer energy. This method also produces a certain amount of turbulence in the stream flows.
Attempts at promoting mixing such as introduction of discontinuities and impingement of the flows also produces turbulence. Radial injection, commonly used with thermal spray processes such as plasma to ensure mixing in a short distance also produces turbulence as the two streams intersect at right angles. In fact, most acceptable methods of injection that promote rapid mixing currently use methods that deliberately introduce turbulence as the means to promote the mixing. The turbulence serves to break down the boundary layer between the flows and once this is accomplished mixing can occur.
The additional turbulence often results in unpredictable energy transfer between the effluent and particulate bearing carrier stream as the flow field is constantly in flux, producing variations within the flow field that affect the transfer of energy. Turbulence represents a chaotic process and causes the formation of eddies of different length scales. Most of the kinetic energy of the turbulent motions is contained in the large scale structures. The energy “cascades” from the large scale structures to smaller scale structures by an inertial and essentially inviscid mechanism. This process continues creating smaller and smaller structures which produces a hierarchy of eddies. Eventually this process creates structures that are small enough that molecular diffusion becomes important and viscous dissipation of energy finally takes place. The scale at which this happens is the Kolmogorov length scale. Thus the turbulence results in conversion of some of the kinetic energy to thermal energy. The result is a process that produces more thermal energy rather than kinetic for transfer to the particles, limiting the performance of such devices. Complicate the process by having more than one turbulent stream and the results are unpredictable as stated.
Turbulence also increases energy loss to the surroundings as the turbulence results in loss of at least some of the boundary layer in the effluent flow field and thus promotes the transfer of energy to the surroundings as well as frictional affects within the flow when flows are contained within walls. For flow in a tube the pressure drop for a laminar flow is proportional to the velocity of the flow while for turbulent flow the pressure drop is proportional to the square of the velocity. This gives a good indication of the scale of the energy loss to the surroundings and internal friction.
Thus there remains a need in the art for an improved method and apparatus to promote rapid mixing of axially injected matter into thermal spray process guns and also limits the generation of turbulence in the flow streams as a result.